In optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam into a smooth spectral continuum. There is no definitive explanation of how much spectral broadening constitutes a supercontinuum.
A supercontinuum source typically consists of a pulsed laser and a non-linear element, in which a combination of non-linear effects in the non-linear element broadens the narrow-band, pulsed laser radiation into a continuous spectrum without destroying the spatial coherence of the laser light. Photonic crystal fibers are uniquely suited as the non-linear medium for such supercontinuum sources, offering high non-linearity, suitable dispersion characteristics and ease of use.
Examples of photonic crystal fibers used to create supercontinuum sources are available, from NKT Photonics in Denmark, Fianium in the UK, and Leukos France.
These nonlinear photonic optical fibers are generally engineered to have particular dispersion and nonlinear characteristics based on specific wavelengths of interest. These nonlinear fibers also typically have very small core diameters (<5 micron diameter). The reason for such small diameter cores is to increase the optical intensity in the optical fiber. The smaller the core diameter is, the higher the optical intensity in the fiber, and the lower the nonlinear thresholds are in that fiber for a given length. Thus, it is easier to generate a supercontinuum using a shorter length optical fiber.
An alternative to using a nonlinear optical fiber with a small diameter core is to use one with a larger diameter core and increase the length of the optical fiber. However, using nonlinear optical fibers having long lengths usually leads to greater optical propagation losses, so using optical fibers having small diameter cores helps to maintain a short length of optical fiber to minimize losses and maximize output power.
The main problem with small diameter optical fibers is that it is very difficult to free-space couple a laser beam into an end of the optical fiber and hold the beam thereon efficiently. The tolerances associated with coupling a laser beam into an end of these small diameter optical fibers are very tight, and any slight misalignments can lead to a loss of coupling between the two, or to damage of the optical fiber. If a laser-optical fiber system is required to handle even moderate temperature excursions, this laser beam to optical fiber coupling becomes more difficult and very expensive to overcome.
Using fiber optic cables having larger core diameters offers easier optical coupling between a laser source and an end of the cable with looser tolerances. However, optical fibers having larger diameter cores increases nonlinear thresholds and decreases optical intensity in the fibers. To overcome the decrease in optical intensity the length of the optical fiber must be increased. However, the increase in length of the optical fiber often leads to more propagation losses. In addition, use of optical fibers having larger diameter cores usually results in multimode operation of the optical fiber, which also has an effect on nonlinear thresholds.
Thus, a need therefore exists in the art for an inexpensive, broadband, supercontinuum generator that utilizes standard, larger diameter optical fibers, rather than specialized nonlinear optical fibers having small core diameters.